In a traditional laboratory, instruments described as spectrometers, spectrophotometers or photometers (referred to from here on as spectrometers) are used to make measurements on liquids or solutions containing one or more chemical substances. Such methods of analysis are used to measure the concentration of a component either directly or following the reaction with one or more chemical substances, usually described as reagents. In such reactions the analyte, or material being measured, is converted into a chemical form that can be detected within the spectral region covered by the instrument. Examples can include the formation of a specific color, or the formation of a material that provides a characteristic fluorescence or luminescence, especially in the presence of radiation of specific wavelengths, such as an ultraviolet source, or the formation of a light scattering medium, where the degree of light scatter is proportional to the concentration of the analyte (substance or species being measured). This latter case includes turbidity for the measurement of suspended materials. In certain spectral regions, such as the ultraviolet, near infrared and the mid infrared, materials can have natural absorption characteristics, where the material can be measured directly in the absence of a reagent. Similar situations occur where an analyte is naturally colored or naturally fluorescent. In these situations reagents are not required.
The normal procedure in a laboratory is to prepare the sample for analysis. The circumstances described in the preceding paragraph above are for the measurement of samples in a liquid form. Spectral measurements are not limited to liquids, and samples that exist as solids or gases can be considered for spectroscopic analysis if prepared in a form that can be measured. For most applications involving reagents, a liquid-based medium is implied. Both solids and gases can be handled if dissolved within a reagent system, or if dissolved in a suitable solvent. If the sample has its own natural spectral response, in the absence of a reagent, the sample may be studied in its natural form as a solid or gas. Such measurements require some form of specialized sample handling accessory. Samples existing in the liquid state are often preferred for reasons of convenience of sampling and handling, and because the sample as studied is generally homogenous and representative of the whole sample.
The standard approach to handling liquids is to place the sample with a container with optically transparent walls or windows. Such containers are called a cells or cuvettes (referred to from here on as cells). If the sample must be treated with a reagent prior to analysis then the sample is normally placed in a separate container, such as a laboratory flask or bottle, prior to placement within the cell. Such a preparation can also require heating or an incubation period. Once the sample is transferred into the measurement cell, the cell is placed at a sampling point within the spectrometer. Typically, this sampling point is a chamber or sampling compartment, which is often light-tight, and can be sealed from interference from ambient light. The sampling chamber may be configured to accept one or more sampling cells. In an alternative rendering, the sample cell may be configured for sample flow through the cell. In such systems, a reagent may be introduced in the sample flow, enabling the regent to interact with the sample in situ.
Most laboratory instruments occupy bench space, and as such they can be limited in terms of access. Furthermore, most laboratory instruments are relatively expensive, and so the number of instruments available for use by laboratory personnel may be limited. In recent years, smaller and lower cost instruments have become available, but these can cost several thousands of dollars once they are configured to be a fully functional instrument. Many of the newer generation of instruments utilize fiber optic cables to couple the spectrometer to the sample. While these present some flexibility, they are also constrained by the length of the fibers and the overall lack of flexibility of the cable. All cables and fibers are limited in their flexibility by their bend radius. Also, fiber optics can impose signal quality issues on the collected spectral data that can negatively impact the final results unless careful consideration is given to the way the system is implemented.
In certain industries and for certain applications, such as environmental measurements, it is desirable to make measurements in a non-laboratory environment. Examples can include measurements on water samples taken at an industrial site or from a stream, river or lake for contaminants or undesirable materials. In such cases, the measurements ideally must be made on a portable instrument. In the absence of a portable instrument there is the burden of sending collected samples back to a laboratory for analysis. Most portable instruments still require the use of a cell, and most require samples to be prepared by mixing with reagents followed by a transfer to the cell. This is not always a convenient scenario. The ideal situation would be to sample directly after the reagent is added without the need to transfer to a cell, or if possible to sample directly from the source, where the reagent is introduced as part of the sample handling. Such systems are not currently available for field-based (non-laboratory) sample handling. Thus, while small format instruments exist and are used for standard types of measurements with standard cells, they still possess many of the limitations of traditional instruments. Also, it is normal for most portable instruments to be restricted in performance and perform a small number of fixed analyses.